Model test method for tunnel multi-layer support structure

Abstract

The application discloses a model test method of a tunnel multilayer supporting structure, which comprises the following steps: determining the test nature, geometric similarity ratio, model box size and structure form of the model test; converting the geometric similarity ratio to determine the parameters of surrounding rock grade, anchor rod, spraying layer and secondary lining supporting material; designing the load of a jack, and determining the load application range; manufacturing a rough hole model body of a secondary lining-spraying layer-anchor rock bearing arch working condition; applying anchor rod similar material; applying spraying layer similar material; applying secondary lining similar material; loading in stages until the supporting is damaged; monitoring the stress and strain of surrounding rock and each supporting type, recording the stress data and strain data, obtaining the safety factor of each supporting and calculating the total safety factor. The application studies the supporting effect and bearing capacity of the tunnel multilayer supporting structure, calculates the total safety factor of the tunnel multilayer supporting structure, and promotes the development of a new generation of information technology.

Description

Technical Field

[0001] This invention belongs to the field of tunnel engineering and relates to a model test method for multi-layer support structures in tunnels, and more particularly to a model test method for multi-layer support structures in tunnels based on the total safety factor method design theory. Background Technology

[0002] Since its inception, tunnel engineering has undergone in-depth research in many aspects, including surrounding rock characteristics, surrounding rock pressure, support materials, support mechanisms, interaction between support and surrounding rock, calculation methods for support structures, and treatment of special surrounding rocks and adverse geological conditions. This has resulted in a wealth of engineering experience and design and construction theories.

[0003] Common methods for support structure design include engineering analogy, load-structure method, stratum structure method, and characteristic curve method. Among these, the engineering analogy method cannot yield a definitive safety factor; the safety factor derived from the load-structure method can only be used for secondary linings, and different codes have different load values; the stratum structure method can be used for calculations of various strata and structural shapes, but because the calculation conditions (ground stress, surrounding rock constitutive model) are difficult to determine accurately, it is mainly used for qualitative analysis; the characteristic curve method is mainly used for circular tunnels with λ=1, but because both the surrounding rock characteristic curve and the support characteristic curve are difficult to determine, it is also a qualitative analysis method. In summary, support structure design is still in a stage of "experience-based, calculation-assisted" and cannot achieve quantitative design of support parameters.

[0004] The total safety factor method has initially established a quantitative design system for tunnel support structures, enabling a shift from "analogical design" to "quantitative design" in tunnel support structures, and playing a positive role in promoting the development of tunnel engineering design technology. One complex multi-layered structure in the total safety factor method is a three-layered load-bearing structure consisting of a secondary lining, shotcrete layer, and anchor rock bearing arch, comprising the anchor rock bearing arch, shotcrete layer, and secondary lining. Currently, the total safety factor method has provided a calculation method for the safety factor of this multi-layered structure: first, load-bearing structural models of the shotcrete layer, anchor rock bearing arch, and secondary lining are established separately; then, the safety factors of each are calculated; and finally, the total safety factor of the multi-layered support structure is obtained by combining the safety factors of the three. This represents a new generation of information technology. However, there are currently no specific model tests to verify the theoretical results of this new technology. Summary of the Invention

[0005] To address the aforementioned technical problems in the background art, this invention provides a model test method for multi-layered tunnel support structures. This method involves conducting model tests on a secondary lining-shotcrete-anchor rock bearing arch under two working conditions: Class IV and Class V surrounding rock. The method studies the support effect and bearing capacity of the multi-layered tunnel support structure, calculates the total safety factor of the multi-layered support structure, further improves the total safety factor method design system, and promotes the development of next-generation information technology.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A model test method for a multi-layered support structure of a tunnel, characterized in that the model test method for the multi-layered support structure of a tunnel includes the following steps:

[0008] S1) Determine the experimental nature, geometric similarity ratio, model box dimensions, and structural form of the model test;

[0009] S2) Determine the parameters of the surrounding rock grade, anchor bolts, shotcrete layer, and secondary lining support material based on the geometric similarity ratio converted from the working conditions;

[0010] S3) Based on the design values ​​of Class IV and Class V surrounding rock pressure and the total safety factor in the supporting working conditions, design the load of the jack and determine the range of applied load;

[0011] S4) Construct a rough tunnel model of the secondary lining-sprayed layer-anchor rock bearing arch condition;

[0012] S5) Apply a material similar to the anchor bolt;

[0013] S6) Apply a sprayed layer of similar material;

[0014] S7) Apply a similar secondary lining material;

[0015] S8) Graded loading until support failure;

[0016] S9) Monitor the stress and strain of the surrounding rock and various support types, record the stress and strain data, obtain the safety factor of each support such as shotcrete, secondary lining, and anchor bolts, and calculate the total safety factor of the multi-layer support structure.

[0017] Furthermore, in step S1), the nature of the test is to disregard the interaction between the strata and the surrounding rock, and is solely a test of the structural bearing capacity; the structural form in step S1) is selected as an arch structure.

[0018] Furthermore, the geometric similarity ratio used in step S1) is 12.5; the length × width × height of the model box in step S1) is 2.08m × 0.45m × 1.98m.

[0019] Furthermore, the method for determining the surrounding rock grade in step S2) is as follows:

[0020] a) Obtain similar materials for the surrounding rock by converting the geometric similarity ratio based on the mechanical parameters of the surrounding rock under the working condition; the similar materials for the surrounding rock are based on the unit weight γ, deformation modulus E, and internal friction angle. And the mechanical parameter with cohesion c as a reference;

[0021] b) Determine the proportions of aggregate, binder, and modifier based on the composition and proportions of similar surrounding rock materials;

[0022] c) Prepare surrounding rock similar materials according to the ratio of aggregate, binder and modifier;

[0023] d) Conduct uniaxial compressive strength and direct shear tests on the surrounding rock similar materials, and determine the Class IV and Class V surrounding rock similar materials based on the test results;

[0024] The anchor parameters in step S2) include tensile stiffness EA and tensile strength σ. t And the elastic modulus E;

[0025] The sprayed layer in step S2) includes sprayed concrete, steel mesh, and steel arch; the steel arch is simulated by steel sheets.

[0026] In step S2), the secondary lining is made of concrete, and the concrete has a compressive strength σ. c And the elastic modulus E.

[0027] Furthermore, the specific implementation method of step S3) is as follows:

[0028] S31) Determine the burial depth corresponding to Class IV surrounding rock and Class V surrounding rock based on the working conditions;

[0029] S32) Refer to the design value of the surrounding rock pressure in the working condition and the total safety factor method to calculate the maximum load that the similar material of Class IV surrounding rock needs to bear and the maximum load that the similar material of Class V surrounding rock needs to bear respectively.

[0030] S33) Based on the results obtained in step S32), calculate the load applied by the jack in the Class IV surrounding rock similar material and the load applied by the jack in the Class V surrounding rock similar material respectively; the load applied by the jack in the Class IV surrounding rock similar material is the product of the maximum load that the Class IV surrounding rock similar material needs to bear and the load expansion factor; the load applied by the jack in the Class V surrounding rock similar material is the product of the maximum load that the Class V surrounding rock similar material needs to bear and the load expansion factor.

[0031] S34) Based on the calculation results of step S33), apply corresponding loads to the vertical and horizontal directions of the model box.

[0032] Furthermore, the specific implementation method of step S4) is as follows:

[0033] S41) Lay and compact similar materials to the surrounding rock;

[0034] S42) Based on similar materials for the surrounding rock, the tunnel mold is prefabricated and embedded, and monitoring instruments are pre-embedded in the tunnel mold; the tunnel mold is made of polystyrene foam material and wrapped with a plastic film on the outside of the model;

[0035] S43) Excavate the tunnel mold in stages, and at the same time remove the plastic film to obtain the rough tunnel model body of the secondary lining-sprayed layer-anchor rock bearing arch working condition.

[0036] Furthermore, the specific implementation method of step S5) is as follows:

[0037] S51) Select similar materials for the anchor bolts based on the anchor bolt parameters;

[0038] S52) A groove is dug in a material similar to the anchor bolt, and fiber optic strain measurement points are placed in the groove and then encapsulated.

[0039] S53) After the tunnel mold is excavated in stages, holes are drilled at predetermined locations according to the distribution of anchor bolts. The anchor bolts are set around the tunnel wall of the model. Epoxy resin bonding material is injected into the drilled holes using a grouting machine. Then, a tray-like material is bonded to the tunnel wall near the rough tunnel model (after drilling, the anchor bolt-like material is inserted into the hole, and then the gasket on the anchor bolt head, i.e., the tray, is put on the anchor bolt and glued. The tray is the gasket on the anchor bolt head and is used after the anchor bolt is driven into the surrounding rock. The tray is part of the anchor bolt-like material).

[0040] The specific implementation method of further step S6) is as follows:

[0041] S61) Select a similar material for the spray layer based on the spray layer parameters;

[0042] S62) Several miniature earth pressure cells are fixed inside the perimeter of the hole model with tape or grout to measure the contact pressure between the surrounding rock and the sprayed layer.

[0043] S63) Apply a similar spray layer to the perimeter of the hole in the hole model, wherein the thickness of the similar spray layer is 2.0 cm;

[0044] S64) Prepare small pieces of spray-like material for bonding strain gauges and attach them into the coating material;

[0045] After the S65 coating is applied, it is air-dried and baked. After it is completely air-dried, strain gauges are attached to the inside of the sprayed layer.

[0046] The specific implementation method of further step S7) is as follows:

[0047] S71) Select similar materials for the secondary lining based on the secondary lining parameters;

[0048] S72) Strain gauges are attached to the inner and outer sides of the precast secondary lining;

[0049] S73) The entire outer perimeter of the secondary lining is wrapped with a smooth plastic film to simulate a waterproof layer;

[0050] S74) Arrange steel arch-like materials on the outside, and then place the secondary lining into the hole of the burr hole model;

[0051] S75) Grouting machines are used to fill the voids between the secondary lining and the periphery of the cavities in the cavities model.

[0052] The specific implementation method of further step S8) is as follows:

[0053] S81) Select the loading device;

[0054] S82) Perform a loading test on the results obtained in step S7) using a loading device;

[0055] S83) Determine whether the loading test is completed. If yes, proceed directly to step S9; otherwise, continue to step S82) until the loading test is completed. The termination condition of the loading test is that the secondary lining material shows large-scale cracks and crushing phenomena.

[0056] Furthermore, the specific implementation method of step S9) is as follows: monitor the stress and strain of the surrounding rock and each type of support, observe the failure mode and failure sequence, record the ultimate bearing capacity and strain data of the multi-layer support structure, and obtain the safety factor of each support and the total safety factor.

[0057] Compared with the current indoor model test methods and schemes for multi-layer support structures in tunnels, the beneficial effects of this invention are:

[0058] The multi-layer support structure described in this invention is a three-layer composite structure consisting of an anchor-rock bearing arch (hereinafter referred to as "anchor-rock bearing arch"), a shotcrete layer (including shotcrete, steel mesh, and steel arch frame, hereinafter referred to as "shotcrete layer"), and a secondary lining (hereinafter referred to as "secondary lining"). This invention involves conducting a scaled-down test on the original tunnel. The steps include determining the nature of the test, establishing a geometric similarity ratio, determining the size of the model box and the structural form of the test, then determining the parameters of similar materials for the surrounding rock, anchors, shotcrete layer, and secondary lining based on the geometric similarity ratio and the working conditions. After fabricating the mold, it is placed in the model frame, subjected to graded loading, and the stress and strain of the surrounding rock and each support type are monitored. The ultimate bearing capacity of the multi-layer support structure is recorded, and the safety factor of the support is calculated based on the collected data.

[0059] This invention addresses the challenge that in most current model tests, the dimensions of similar materials used in the secondary lining, shotcrete layer, and anchor bolts are relatively small, making it difficult to observe and monitor their load-bearing effects. In contrast, this experimental method uses a larger support structure, specifically an arch structure, which better reflects the load-bearing mechanism and load effect of the support in the results. Furthermore, while existing model tests calculate the safety factor of multi-layered tunnel support structures, this experimental method calculates both the safety factor and the overall safety factor of the model, further refining the overall safety factor method design system.

[0060] The indoor model testing method employed in this invention is an auxiliary testing method that scales up or down an actual model to maintain similarity to the original structure. By scaling down or up, the influence of adverse factors can be avoided, testing costs can be reduced, and the cycle can be shortened. Therefore, model testing can provide experimental verification for some new structural design theories and test the reliability of hypotheses. Attached Figure Description

[0061] Figure 1 Schematic diagram of a multi-layer support structure.

[0062] Figure 2 Model box dimensions diagram.

[0063] Figure 3 Step-by-step diagram for making the fuzzy hole model.

[0064] Figure 4 Layout diagram of monitoring elements inside and outside the load-bearing arch.

[0065] Figure 5 Image showing the effect of applying a material similar to that used for anchor bolts.

[0066] Figure 6 Image showing the effect of applying a similar sprayed layer of material.

[0067] Figure 7 Image showing the effect of applying a similar secondary lining material.

[0068] Figure 8 Model test loading test bench diagram; where (a) is the front view of the bench and (b) is the rear view of the bench. Detailed Implementation

[0069] The technical solutions in the embodiments of the present invention will be further described in detail with reference to the accompanying drawings. The scope of the present invention is not limited thereto.

[0070] Example

[0071] The model test provided by this invention is based on the optimized test section of the Yichang-Zhengwan High-speed Railway connecting line. The tunnel adopts a double-track tunnel cross-section with a speed of 350km / h, an inner profile height of 10.88m, and a span of 13.3m.

[0072] The tunnel's multi-layered support structure comprises a three-layered composite structure consisting of a rock-supported arch, a shotcrete layer, and a secondary lining. Its structure is as follows: Figure 1 As shown.

[0073] Specifically, the present invention provides a model test method for a multi-layered support structure of a tunnel, comprising the following steps:

[0074] S1, determine the experimental nature, geometric similarity ratio, model box size, and structural form of the model test.

[0075] S1 includes the following sub-steps:

[0076] The test is a structural bearing capacity test. In existing tunnel model tests, most tests use a geometric similarity ratio of over 50, with cross-sectional dimensions ranging from approximately 20cm to 60cm, generally following Saint-Venant's principle. When the geometric similarity ratio is large, the stress and failure modes of various support components, such as linings and anchors, are difficult to monitor due to the relatively small cross-sectional dimensions. However, the focus of this test is to analyze the stress characteristics of the tunnel support structure. Under the condition that the surrounding rock material does not collapse after excavation, the tunnel cross-section should be as large as possible to facilitate observation of support failure and monitoring of the stress on support components. Furthermore, the application of boundary loads has a significant impact on the support stress, so the geometric similarity ratio should be set relatively small. The geometric similarity ratio used in this test is 12.5. Since the experiment mainly examines the mechanical failure characteristics of the tunnel structure during loading, the boundary is to be relatively small. Based on the previously determined geometric similarity ratio (12.5), and considering the prototype tunnel's span of 13.3m and height of 10.88m in the working condition, the model tunnel's span is 1.06m and height is 0.87m. Taking into account factors such as the tunnel excavation impact range, economy and technology, ease of construction for personnel, and plane strain, the model test box is preliminarily determined to be 2.08m × 0.45m × 1.98m (length × width × height). An arch structure is adopted, placed inside the 2.08m × 0.45m × 1.98m (length × width × height) model box. Figure 2 The advantage of arch structures is that they can demonstrate the load-bearing capacity of an arch, making them more similar to structural load tests and theoretical models.

[0077] S2, determine the parameters of surrounding rock grade, anchor bolts, shotcrete layer, and secondary lining support materials based on the geometric similarity ratio converted according to the working conditions.

[0078] S2 includes the following sub-steps:

[0079] S21, similar materials for surrounding rock are characterized by specific gravity γ, deformation modulus E, and internal friction angle. Cohesion c was used as the main reference mechanical parameter. The target parameters of the surrounding rock similarity material were obtained by converting the mechanical parameters of the surrounding rock under the working condition into geometric similarity ratios, as shown in Table 1. Based on the composition and proportion of the similar materials investigated, the approximate range of the proportions of aggregate, binder, and modifier were determined. Multiple sets of similar materials were prepared on a trial basis, and a large number of uniaxial compressive strength and direct shear tests were carried out. The proportions for achieving the target parameters for Class IV and Class V surrounding rock were basically determined.

[0080] Table 1 Target parameters of similar materials for surrounding rock

[0081]

[0082] S22, anchor bolt parameters, including tensile stiffness EA and tensile strength σ. t The elastic modulus E is the main research object. According to the similarity ratio calculation theory, when the geometric similarity ratio is 12.5, the similarity ratios of tensile strength and elastic modulus are also 12.5, while the tensile stiffness needs to be recalculated twice, and its final similarity ratio is 1953. Through pull-out tests of different types of materials, the target parameters of the anchor rod similar materials are obtained, as shown in Table 2.

[0083] Table 2 Target parameters of similar materials for anchor bolts

[0084]

[0085] S23. Based on the parameters of the sprayed layer and the geometric similarity ratio in the working condition, the similar materials of the sprayed layer are calculated. The sprayed layer includes shotcrete, steel mesh and steel arch frame. The steel arch frame is simulated by steel sheets. The I-beam I20a involves many mechanical parameters. The elastic modulus is used as the main mechanical parameter to be examined. According to the calculation, the target parameters of the sprayed layer are shown in Table 3.

[0086] Table 3 Target parameters of similar materials for the sprayed coating

[0087]

[0088]

[0089] S24, based on the parameters and geometric similarity ratio of the secondary lining under the supporting working conditions, the target parameters of the secondary lining material are calculated. The secondary lining is mainly composed of concrete, with compressive strength σ c Using the elastic modulus E as the main mechanical parameter, uniaxial compressive strength tests were conducted on similar materials for the secondary lining. The proportions of similar materials for the secondary lining were initially determined, and the target parameters for the secondary lining are shown in Table 4.

[0090] Table 4 Target parameters of similar materials for secondary lining concrete

[0091]

[0092] S3, determine the range of applied loads based on the load of the jack designed for the working conditions.

[0093] The load design for the jack should consider a geometric similarity ratio of 12.5, referencing the design values ​​of surrounding rock pressure in the supporting working conditions (165 kPa for Class IV surrounding rock at a depth of 800 m, and 478 kPa for Class V surrounding rock at a depth of 600 m), and the overall safety factor (11.62 for Class IV and 4.63 for Class V) to determine the maximum load that similar materials of different grades of surrounding rock need to withstand.

[0094] Class V surrounding rock: 478 kPa × 4.63 / 12.5 = 0.1771 MPa

[0095] Class IV surrounding rock: 165 kPa × 11.62 / 12.5 = 0.1534 MPa

[0096] Since the model test is used to study the ultimate bearing capacity of the support structure, the load applied by the jack in the model test is set as the maximum load multiplied by the corresponding load amplification factor, which is 4 times.

[0097] Class V surrounding rock: 0.1771 MPa × 4 = 0.7084 MPa

[0098] Class IV surrounding rock: 0.1534 MPa × 4 = 0.6136 MPa

[0099] Therefore, the maximum load required for the test is 0.7084 MPa.

[0100] Required jack tonnage in vertical and horizontal directions:

[0101] 2.08m×0.45m×0.7084Pa / 9.8N / kg=67.66T

[0102] 1.98m × 0.45m × 0.7084MPa / 9.8N / kg × 0.6 = 38.64T (taking the lateral pressure coefficient as 0.6)

[0103] Based on the vertical and horizontal loads, a row of jacks is set on each of the three sides of the model box: the top, left, and right. There are a total of four 30T jacks in each row on the top side, and four 30T jacks in each row on the left and right sides.

[0104] Vertical jacks can provide loads:

[0105] 120000kg × 9.8N / kg / (2.08m × 0.45m) = 1.256MPa (7 times the design load)

[0106] A horizontal jack can provide the following load:

[0107] 120000kg × 9.8N / kg / (1.98m × 0.45m) = 1.320MPa (7.4 times the design load)

[0108] S4, Specific Implementation Plan for the Fabrication of a Model of a Cavity under the Condition of Secondary Lining-Shotcrete-Anchor Rock Bearing Arch

[0109] S4 includes the following sub-steps:

[0110] S41, Production as follows Figure 1 The model box measures 2.08m × 0.45m × 1.98m (length × width × height). The internal model tunnel dimensions are calculated using a geometric similarity ratio of 12.5, equivalent to the prototype tunnel in the working conditions. Therefore, the model tunnel has a span of 1.06m and a height of 0.87m.

[0111] S42, the construction of the pore model body adopts the layered spreading and compaction method. Figure 3 To create a rendering of the distribution, the specific process is as follows:

[0112] S421) Weigh and prepare the surrounding rock similar material on a large scale according to the specified material ratio. Mix the material evenly with a mixer to determine the number of material layers and the amount of material needed for each layer. Pour the evenly mixed bottom similar material into the bottom of the model frame and spread it evenly. Compact it according to the specified compaction degree. Use a high-powered electric fan to air-dry the compacted material layer by layer, ensuring that the solvent in the model body completely evaporates. Figure 3 As shown in (a).

[0113] During the process of laying similar materials, a 3mm thick polytetrafluoroethylene (PTFE) film needs to be prefabricated on all four sides. The PTFE film has the characteristics of high lubricity and non-adhesion, which can reduce the friction of the model body.

[0114] (S422) Tunnel mold installation: The tunnel mold is made of a material with certain deformation resistance and easy cutting and excavation. In this experiment, polystyrene foam was selected as the tunnel mold. For example... Figure 3 As shown in (b).

[0115] S423) Install measuring elements (including miniature earth pressure cells, displacement gauges, strain gauges, etc.) at the design elevation. For example... Figure 4 Earth pressure cells around the arch ring are used to monitor the force transmitted through fine sand to the periphery of the arch ring, serving as the external load for load-structure method calculations. Earth pressure cells and strain gauges inside the load-bearing arch are used to measure internal stress and strain, thereby analyzing the mechanical mechanism of the load-bearing arch. Install baffle supports. (As shown) Figure 3 As shown in (c).

[0116] (S424) Fill the area enclosed by the baffle, model box frame, and tunnel mold with similar material and compact it to the specified degree. Use a high-powered electric fan again to air-dry the compacted material layer by layer, ensuring the solvent in the model body completely evaporates. For example... Figure 3 As shown in (d).

[0117] (S425) Remove the baffle and its support, and fill the area enclosed by the baffle, model box frame, and tunnel mold with fine sand. For example... Figure 3 As shown in (e).

[0118] S426) Stretch the support length and install baffle supports on both sides of the tunnel mold. For example... Figure 3 As shown in (f).

[0119] S427) Repeat steps S423), S424), and S425) until the top of the model is reached, then cover it with a steel plate. Repeat this process of filling and compacting 16 layers of similar surrounding rock material and fine sand. Figure 3 As shown in (g).

[0120] S428) Excavate the tunnel mold in stages and remove the outer plastic film from the mold. If the surrounding rock material is unstable during excavation, a thin layer of lining material can be applied around the tunnel, several miniature earth pressure cells can be fixed around the tunnel, and then the entire area around the tunnel can be wrapped with a smooth plastic film to simulate a waterproof layer. Figure 3 As shown in (h).

[0121] S5, apply anchor bolt-like material, excavate grooves in the anchor bolt-like material to insert fiber optic strain gauges and encapsulate them. After excavating the tunnel mold in stages and stabilizing for a period of time, drill holes at predetermined locations according to the anchor bolt layout diagram. Use a grouting machine to inject epoxy resin bonding material into the drill holes, and then bond a tray-like material near the tunnel wall. Figure 5 As shown.

[0122] S6. Apply a similar sprayed layer material, which includes shotcrete, steel mesh, and a steel arch frame. The steel arch frame is simulated using steel sheets. I20a I-beams involve many mechanical parameters, with the modulus of elasticity being the primary consideration. They are arranged in the model at 8cm longitudinal intervals.

[0123] During the application of the shotcrete layer, several miniature earth pressure cells are fixed inside the tunnel perimeter using tape or grout to measure the contact pressure between the surrounding rock and the shotcrete layer. Small pieces of a shotcrete-like material, approximately 2.0 cm thick, are prepared and fixed inside the tunnel perimeter. Then, the shotcrete-like material is applied around the tunnel perimeter (this facilitates monitoring the strain inside and outside the shotcrete layer and can also be used as a calibration of the shotcrete thickness). After application, the material is air-dried. Once completely dry, a steel arch is installed inside the shotcrete layer, and strain gauges are then attached. Figure 6 As shown.

[0124] S7. Apply a similar material to the secondary lining. First, prefabricate the secondary lining mold, which consists of an inner mold and an outer mold. Clean the outer surface of the inner mold and the inner surface of the outer mold, then apply a release agent. Assemble the mold and place it on the mold base to complete the model setup. Pour a specific ratio of plaster and water into the mold in stages. After drying, remove the mold and bake the specimen.

[0125] When applying the secondary lining, strain gauges are attached to both the inner and outer sides of the precast secondary lining. A smooth plastic film is then laid on the secondary lining, except for the invert arch, to simulate a waterproof layer. A similar material to a steel arch frame is then placed on the outer side, and the secondary lining is inserted into the tunnel. The outer contour dimensions of the secondary lining should be designed to be small to ensure easy insertion. Grouting pipes and holes are pre-reserved between the secondary lining and the tunnel perimeter. A grouting machine is used to fill any voids between the secondary lining and the tunnel perimeter. Figure 7 .

[0126] S8, graded loading until support failure.

[0127] S8 includes the following sub-steps:

[0128] The S81 loading device consists of a loading frame, a model box, a hydraulic loading system, a stress-strain measurement system, and a computer and control system. For example... Figure 8 .

[0129] S82, using jacks for graded loading, the loading process was slow and gradual to ensure a quasi-static process. Monitoring equipment was activated to record the stress and strain of the surrounding rock and lining during loading. The loading levels were determined, and the self-weight stress field was simulated based on the geometric similarity ratio. To better ensure the force transmission effect of the similar materials, the test was set to approximately 10 levels each time, increasing by 10% per level, with each level held for 3-5 minutes, based on the measured strain and earth pressure data. Considering the stress state of the surrounding rock material, the test was terminated when large-scale cracks and crushing phenomena appeared in the secondary lining material.

[0130] S9 monitors the stress and strain of the surrounding rock and various support types; observes the failure modes and failure sequence, and explores the bearing mechanism of multi-layer support structures. It records the ultimate bearing capacity of the multi-layer support structure and finally obtains the safety factor of each layer of support structure and the total safety factor of the multi-layer support structure based on the content recorded in "Total Safety Factor Method for Tunnel Support Structure Design" (written by Xiao Mingqing, published by People's Communications Press Co., Ltd., ISBN: 9787114165580).

[0131] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the invention.

Claims

1. A model test method for a multi-layered support structure of a tunnel, characterized in that: Includes the following steps: S1) Determine the experimental nature, geometric similarity ratio, model box dimensions, and structural form of the model test; S2) Determine the parameters of the surrounding rock grade, anchor bolts, shotcrete layer, and secondary lining support material based on the geometric similarity ratio converted from the working conditions; The method for determining the surrounding rock grade in step S2) is as follows: a) Obtain similar materials for the surrounding rock by converting the geometric similarity ratio based on the mechanical parameters of the surrounding rock under the working condition; the similar materials for the surrounding rock are defined by the unit weight γ, deformation modulus E, and internal friction angle. φ And the mechanical parameter with cohesion c as a reference; b) Determine the proportions of aggregate, binder, and modifier based on the composition and proportions of similar surrounding rock materials; c) Prepare surrounding rock similar materials according to the ratio of aggregate, binder and modifier; d) Conduct uniaxial compressive strength and direct shear tests on the surrounding rock similar materials, and determine the Class IV and Class V surrounding rock similar materials based on the test results; The anchor parameters in step S2) include: tensile stiffness EA, tensile strength, etc. And the elastic modulus E; The sprayed layer in step S2) includes: sprayed concrete, steel mesh, and steel arch; the steel arch is simulated by steel sheets. In step S2), the secondary lining is made of concrete, and the concrete includes compressive strength... With elastic modulus E; S3) Based on the design values ​​of Class IV and Class V surrounding rock pressure and the total safety factor in the supporting working conditions, design the load of the jack and determine the range of applied load; S4) Construct a rough tunnel model of the secondary lining-sprayed layer-anchor rock bearing arch condition; S5) Apply a material similar to the anchor bolt; S6) Apply a sprayed layer of similar material; S7) Apply a similar secondary lining material; S8) Graded loading until support failure; S9) Monitor the stress and strain of the surrounding rock and each type of support, record the stress and strain data, obtain the safety factor of each support and calculate the total safety factor.

2. The model test method for multi-layer support structures in tunnels according to claim 1, characterized in that: In step S1), the test does not consider the interaction between the strata and the surrounding rock, and is only a structural bearing capacity test; the structural form is selected as an arch structure.

3. The model test method for multi-layer support structures of tunnels according to claim 1, characterized in that: The specific implementation method of step S3) is as follows: S31) Determine the burial depth corresponding to Class IV surrounding rock and Class V surrounding rock based on the working conditions; S32) Refer to the design value of surrounding rock pressure and the total safety factor method in the working condition, calculate the maximum load that similar materials of Class IV surrounding rock need to bear and the maximum load that similar materials of Class V surrounding rock need to bear respectively. S33) Based on the results obtained in step S32), calculate the load applied by the jack in the Class IV surrounding rock similar material and the load applied by the jack in the Class V surrounding rock similar material respectively; the load applied by the jack in the Class IV surrounding rock similar material is the product of the maximum load that the Class IV surrounding rock similar material needs to bear and the load expansion factor; the load applied by the jack in the Class V surrounding rock similar material is the product of the maximum load that the Class V surrounding rock similar material needs to bear and the load expansion factor. S34) Based on the calculation results of step S33), apply corresponding loads to the vertical and horizontal directions of the model box.

4. The model test method for multi-layer support structures of tunnels according to claim 1, characterized in that: The specific implementation method of step S4) is as follows: S41) Lay and compact similar materials to the surrounding rock; S42) Based on similar materials for the surrounding rock, the tunnel mold is prefabricated and embedded, and monitoring instruments are pre-embedded in the tunnel mold; the tunnel mold is made of polystyrene foam material and wrapped with a plastic film on the outside of the model; S43) The tunnel mold is excavated in stages, and the plastic film is removed at the same time to obtain the rough tunnel model body of the secondary lining-sprayed layer-anchor rock bearing arch working condition.

5. The model test method for multi-layer support structures of tunnels according to claim 1, characterized in that: The specific implementation method of step S5 is as follows: S51) Select similar materials for the anchor bolts based on the anchor bolt parameters; S52) A groove is dug in a material similar to the anchor bolt, and fiber optic strain measurement points are placed in the groove and then encapsulated. S53) After the tunnel mold is excavated in stages, holes are drilled at predetermined positions according to the distribution of anchor bolts. The anchor bolts are set around the tunnel wall of the model. Epoxy resin bonding material is injected into the drilled holes using a grouting machine. Then, a similar material to the tray is bonded to the tunnel wall near the rough tunnel model.

6. The model test method for multi-layer support structures of tunnels according to claim 1, characterized in that: The specific implementation method of step S6) is as follows: S61) Select a similar material for the spray layer based on the spray layer parameters; S62) Several miniature earth pressure cells are fixed inside the perimeter of the hole model with tape or grout to measure the contact pressure between the surrounding rock and the sprayed layer. S63) Apply a similar spray layer to the perimeter of the hole in the hole model, wherein the thickness of the similar spray layer is 2.0 cm; S64) Prepare small pieces of spray-like material for bonding strain gauges and attach them into the coating material; After the S65 coating is applied, it is air-dried and baked. After it is completely air-dried, strain gauges are attached to the inside of the spray layer.

7. The model test method for multi-layer support structures of tunnels according to claim 1, characterized in that: The specific implementation method of step S7) is as follows: S71) Select similar materials for the secondary lining based on the secondary lining parameters; S72) Attach strain gauges to the inner and outer sides of the precast secondary lining; S73) The entire outer perimeter of the secondary lining is wrapped with a smooth plastic film to simulate a waterproof layer; S74) Arrange steel arch-like materials on the outside, and then place the secondary lining into the hole of the burr hole model; S75) Grouting machine is used to fill the voids between the secondary lining and the periphery of the hole model.

8. The model test method for multi-layer support structures of tunnels according to claim 1, characterized in that: The specific implementation method of step S8) is as follows: S81) Select the loading device; S82) Perform a loading test on the model obtained in step S7) using a loading device; S83) Determine whether the loading test is completed. If yes, proceed directly to step S9; otherwise, continue to step S82) until the loading test is completed. The termination condition of the loading test is that the secondary lining material shows large-scale cracks and crushing phenomena.

9. The model test method for multi-layer support structures of tunnels according to claim 1, characterized in that: The specific implementation method of step S9) is as follows: monitor the stress and strain of the surrounding rock and each type of support, observe the failure mode and failure sequence, record the ultimate bearing capacity and strain data of the multi-layer support structure, obtain the safety factor of each support and calculate the total safety factor.

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